Hydraulic mount having double idle rate dip frequencies of dynamic stiffness

ABSTRACT

An hydraulic mount providing double idle rate dip frequencies of dynamic stiffness, the idle rate dips being vibration orders of an internal combustion engine corresponding to one and two times firing frequencies. The double idle rate dip is achieved by providing the first idle rate dip at the first vibration order via resonance tuning of the idle inertia track, and providing the second idle rate dip at the second vibration order via additional resonance tuning of a tunable air conduit.

TECHNICAL FIELD

The present invention relates to hydraulic mounts, as for example usedfor engine mounts in motor vehicle applications, and more particularlyto an hydraulic mount having a pair of idle rate dip frequencies ofdynamic stiffness.

BACKGROUND OF THE INVENTION

Hydraulic mounts are dual aspect devices. In a first aspect, anhydraulic mount provides location of one object, such as a motor vehicleengine, with respect to a second object, as for example the frame of themotor vehicle. In a second aspect, the hydraulic mount provides dampingof vibration or low dynamic stiffness as between the first and secondobjects, as for example damping or isolating of engine vibration withrespect to the frame of the motor vehicle.

Hydraulic mounts which are used for motor vehicle applications arerepresented, for example, by U.S. Pat. Nos. 4,828,234, 5,215,293 and7,025,341.

U.S. Pat. No. 5,215,293 discloses an hydraulic mount having a rigidupper member which is bolted to the engine and a lower engine memberwhich is bolted to the frame, wherein the upper and lower members areresiliently interconnected. The upper member is connected to a resilientmain rubber element. Vibration of the main rubber element in response toengine vibration is transmitted to an adjoining upper fluid chamber. Theupper fluid chamber adjoins a rigid top plate having an idle inertiatrack therethrough which communicates with an idle fluid chamber. Theidle fluid chamber is separated from an idle air chamber by an idlediaphragm. The idle air chamber is selectively connected to atmosphereor to engine vacuum in order to selectively evacuate the idle airchamber in which case the idle diaphragm is immobilized. A bounceinertia track is formed in the top plate and communicates with a lowerfluid chamber which is fluid filled. A bellows separates the lower fluidchamber from a lower air chamber which is vented to the atmosphere.

The idle inertia track has a larger cross-sectional area and a shorterlength than that of the bounce inertia track, such that the ratioprovides resonant frequency damping at the respectively selectedresonance frequencies. In this regard, the resonance frequency of thefluid flowing through the idle inertia track is set to be higher thanthat of the fluid flowing through the bounce inertia track. As such,this prior art hydraulic mount is able to effectively damp relativelylow frequency vibrations over a lower frequency range, such as engineshake or bounce, based on resonance of a mass of the fluid in the bounceinertia track, while, on the other hand, the idle inertia track is tunedso that the hydraulic mount exhibits a sufficiently reduced dynamicstiffness with respect to relatively high-frequency vibrations over ahigher frequency range, such as engine idling vibrations, based on theresonance of a mass of the fluid in the idle inertia track.

In operation, vibrations in the higher frequency range are isolated byoperation of the induced fluid oscillations in the upper fluid chamberpassing through the idle inertia track and the resilient deformation ofthe main resilient element and the idle diaphragm in that the idle airchamber is at atmospheric pressure. For vibrations in the lowerfrequency range, the idle air chamber is evacuated by being connected toengine vacuum, wherein now the fluid oscillations of the upper fluidchamber travel through the bounce inertia track and are damped therebyin combination with the resilient deformation of the main resilientelement and the bellows.

While hydraulic mounts work well, there is the problem that each of thetracks, the idle inertia track and the bounce inertia track, provide lowdynamic stiffness and damping at a respective predetermined range offrequency of vibration. In particular with respect to the idle inertiatrack, the “idle rate dip” frequency is singular, being selected togenerally suit a particular engine. For example as shown at FIG. 4B,which is a graph 300′ of dynamic stiffness of a prior art hydraulicmount versus engine vibration frequency (discussed further hereinbelow),wherein the plot 302′ has a singular idle rate dip 304′ occurring at afrequency of 50 Hz.

Engine idle vibration has more than one order (vibrations perrevolution). For example, it is well known physics that the undampednatural frequency, f_(n), for a simple mass-spring system is given by:

$\begin{matrix}{f_{n} = {\frac{1}{2\;\pi}{\sqrt{k/m}.}}} & (1)\end{matrix}$where k is the spring stiffness and m the sprung mass.

Aspects of internal combustion engines which are relevant to the designof hydraulic mounts therefor include:

-   -   1. the dynamic stiffness K* (complex dynamic stiffness);    -   2. the elastic modulus (elastic dynamic stiffness) K′;    -   3. the out-of-phase modulus (loss modulus, or loss dynamic        stiffness) K″, where term K*=√(K″²+K′²);    -   4. the loss angle θ=Tan⁻¹(K″/K′);    -   5. the damping coefficient (C), where tan θ=Cω/K′, where ω=2πf,        and where f=frequency;    -   6. the resonance frequency of the fluid column in the inertia        track which is the frequency at which the out-of-phase module        (K″) reaches a maximum;    -   7. the rate (dynamic stiffness) dip frequency, which is the        frequency at which the K* reaches a minimum, wherein the rate        dip frequency is several Hz (frequency) lower than the maximum        K″ frequency;    -   8. the are two zones for noise and vibration (N and V): control        and isolation;    -   9. the control zone, which is the frequency below 1.414×f_(n)        (natural frequency), wherein in the control zone, damping is        required to reduce the vibration, engine bounce and shake or        rough road shake (e.g., at low frequency—control zone) requires        high damping to reduce the vibration;    -   10. the isolation zone, which is the frequency above        1.414×f_(n), wherein in the isolation zone, low dynamic        stiffness is required to isolate the vibration, engine idle        shake (e.g., at high frequency—isolation zone) requires low        dynamic stiffness to isolate the vibration, wherein the        isolation zone, the damping increases the vibration;    -   11. the natural frequency of the fluid in the inertia track,        which depends on the mass of the fluid in the track and the        stiffness (bulge/volume stiffness of the upper chamber and lower        chamber); and    -   12. the engine RPM at idle, for example at 900 RPM, the first        order frequency is 15 Hz (e.g., 900/60=15), and wherein the        engine firing orders/frequencies of different engines are shown        in the following Table I.

TABLE I Engine 1x firing 2x firing 3x firing . . . I4 2^(nd) order (30Hz) 4^(th) order 6^(th) order. . . V6 3^(rd) order 6^(th) order 9^(th)order . . . V8 4^(th) order 8^(th) order 12^(th) order . . .

Examples of the one times and two times engine firing frequencies are:for a V6 internal combustion engine, the 3^(rd) and 6^(th) enginevibration orders have the most undesirable vibration, and for a V8engine, the 4^(th) and 8^(th) engine vibration orders have the mostundesirable vibration. The selection of the tuned idle rate (e.g.,dynamic stiffness) dip frequency is, therefore, a best compromiseselection of the frequency of the idle rate dip the one times and twotimes engine firing frequencies, as shown at FIG. 4B.

The vacuum tubing interconnecting with the idle air chamber introducesan undesirable noise factor in the sense of an unwanted dynamicstiffness. Therefore in the prior art, the existence of the vacuumtubing is considered problematic, and there is a general need tointroduce countermeasures to reduce the dynamic stiffness caused by thevacuum tubing, or move the unwanted dynamic stiffness to a frequencyrange which is not under consideration with respect to optimization ofthe idle rate dip.

Accordingly, it would be desirable in the art if somehow a double idlerate dip frequencies of dynamic stiffness could be provided, andfurther, if somehow the unwanted dynamic stiffness of the vacuum tubingcould somehow be eliminated as a problem.

SUMMARY OF THE INVENTION

The present invention is an hydraulic mount which provides double idlerate dip frequencies of dynamic stiffness, each idle rate dip being at arespective vibration order, by converting the noise problem of thevacuum tubing of the prior art into a resonance tunable air conduit ofthe present invention. The double idle rate dip is achieved by providinga first idle rate dip at a one times engine firing frequency byresonance tuning of the idle inertia track, and providing a second idlerate dip at a two times engine firing frequency by additionallyresonance tuning of the tunable air conduit.

The hydraulic mount according to the present invention may have anysuitable configuration, as for example generally as that describedhereinabove with respect to U.S. Pat. No. 5,215,293, in which an uppermember is resiliently interconnected with a lower member, whereinincluded are: a main resilient (i.e., rubber) element which responds toa source (i.e., engine) vibration at the upper member which istransmitted to an adjoining upper fluid chamber. The upper fluid chambercommunicates with an idle fluid chamber via an idle inertia track, andthe idle fluid chamber is separated from an idle air chamber by an idlediaphragm. The idle air chamber is connected to atmosphere by a tunableair conduit of an air conduit assembly, the connection being selectivelyconnectable to atmosphere or to engine vacuum so as to immobilize theidle diaphragm in association with further resonance damping of a bounceinertia track.

According to the present invention, vibration of the upper member withrespect to the lower member is transmitted to the main resilientelement. With the tunable air conduit open to atmosphere, the idleinertia track is resonance tuned, in responsive combination with theresilient deformation of the main resilient element and the idlediaphragm, to provide a first idle rate dip of dynamic stiffness, and,further thereto, the tunable air conduit is resonance tuned, empiricallyor mathematically as for example based upon the physics of air resonancein an open tube, to provide a second idle rate dip, wherein the firstand second idle rate dips are preferably selected to each coincide,respectively, with the vibration orders of the one times and two timesengine firing frequencies.

Accordingly, it is an object of the present invention to provide anhydraulic mount which provides double idle rate dip frequencies ofdynamic stiffness, each idle rate dip being at a respective vibrationorder, wherein the double idle rate dip is achieved by providing a firstidle rate dip at a one times engine firing frequency by resonance tuningof the idle inertia track, and providing a second idle rate dip at a twotimes engine firing frequency by additional resonance tuning of thetunable air conduit.

This and additional objects, features and advantages of the presentinvention will become clearer from the following specification of apreferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, partly sectional side view of an hydraulic mountaccording to the present invention which provides double idle dipresonance frequencies of damping, shown with the idle air chamber andthe tunable air conduit open to the atmosphere through a solenoid valve.

FIG. 1A is a cross-sectional view of the tunable air conduit, seen alongline 1A-1A of FIG. 1.

FIG. 2 is a schematic, partly sectional side view of an hydraulic mountaccording to the present invention, as in FIG. 1, now shown with theidle air chamber and the tunable air conduit connected to engine vacuum.

FIG. 3 is an algorithm for tuning to provide double idle rate dipfrequencies of dynamic stiffness of the hydraulic mount according to thepresent invention.

FIG. 4A is a graph depicting idle dynamic stiffness versus frequency ofthe hydraulic mount according to the present invention, showing doubleidle rate dip frequencies of dynamic stiffness.

FIG. 4B is a graph depicting idle dynamic stiffness versus frequency ofa prior art hydraulic mount, showing a single idle rate dip frequency ofdynamic stiffness.

FIGS. 5A through 5C depict aspects of idle rate dip tuning, wherein 5Atunes, per the prior art, for a second vibration order of an inline fourcylinder (I4) internal combustion (IC) engine, FIG. 5B tunes, per theprior art, for a fourth vibration order for the I4 IC engine, and FIG.5C tunes, according to the present invention, for both the second andfourth vibration orders of the I4 IC engine.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the Drawing, aspects of a double idle rate diphydraulic mount according to the present invention will be detailed.

Referring firstly to FIGS. 1 through 2, an example of a double idle ratedip hydraulic mount 100 is shown, having certain structural aspectsgenerally similar, by way of exemplification, to the disclosure of U.S.Pat. No. 5,215,293; however, it is to be understood the configurationmay be other than that shown and described and that the air conduitassembly 130 as described structurally and functionally hereinbelow isunique to the present invention.

An upper member 102 resiliently interconnects with a lower member 104.The upper member 102 includes a rigid upper mounting member 106,whereby, for example, bolts 106 a provide connection for example to anengine bracket (not shown). The lower member 104 includes a stampedrigid lower housing 108 whereby, for example, bolts 108 a provideconnection for example to the vehicle frame (not shown).

The resilient interconnection between the upper and lower members 102,104 is provided by a main resilient element 110 which is connected tothe upper mounting member 106, the main resilient element being forexample composed of a rubber material. Opposite the upper mountingmember 106, the main resilient element 110 adjoins an upper fluidchamber 112 filled with an hydraulic fluid 114, as for example glycol.The upper fluid chamber is defined by the main resilient element and asealingly interfaced rigid top plate 116. The rigid top plate 116 hasformed therein an idle inertia track 118 and a bounce inertia track 120.

The idle inertia track 118 communicates with an idle fluid chamber 122,wherein the hydraulic fluid 114 of the upper fluid chamber communicateswith and fills the idle inertia track and idle fluid chamber. The idlefluid chamber 122 is separated from an idle air chamber 124 by aresilient idle diaphragm 126, as for example composed of a rubbermaterial. The idle diaphragm is connected to a rigid partition member128 such that idle air chamber 124 is sealingly defined by the idlediaphragm and the partition member. The idle air chamber 124 isselectively connected to atmosphere or to engine vacuum via an airconduit assembly 130, in which a tunable air conduit 140 thereof passesthrough the partition member 128, the structural and functional aspectof the air conduit assembly being discussed in detail hereinbelow.

The bounce inertia track 120 communicates with a lower fluid chamber 132which is filled with the hydraulic fluid 114. A bellows 134, as forexample composed of a rubber material, is connected to the partitionmember 128, and separates the lower fluid chamber from a lower airchamber 136 which is vented to the atmosphere by a vent 138.

An outer housing 160 connects to each of the main resilient element 110,the top plate 116, the partition member 128 and the lower housing 108.

The idle inertia track 118 has a larger cross-sectional area and ashorter length than that of the bounce inertia track 120, such that theratio provides resonant frequency damping at the respectively selectedresonance frequencies. In this regard, the resonance frequency of thehydraulic fluid 114 flowing through the idle inertia track is set to behigher than that of the fluid flowing through the bounce inertia track.As such, the hydraulic mount 100 is able to effectively damp relativelylow frequency vibrations over a lower frequency range, such as engineshake or bounce, based on resonance of a mass of the fluid in the bounceinertia track, while, on the other hand, the idle inertia track is tunedso that the hydraulic mount exhibits a sufficiently reduced dynamicstiffness with respect to relatively high-frequency vibrations over ahigher frequency range, such as engine idling vibrations, based on theresonance of a mass of the hydraulic fluid in the idle inertia track.

The above referenced air conduit assembly 130 includes: the tunable airconduit 140 (part of which passes through the partition member 128); asolenoid valve 142 which is switchable between first and second states,for example by a controller 144 programmed to respond to the operationof the hydraulic mount, and is connected to the tunable conduit at aninlet 142 a thereof, an exhaust 146 connected a first outlet 142 b ofthe solenoid valve; a vacuum tank 148 connected to a second outlet 142 cof the solenoid valve; and a one-way check valve 150 which allowsairflow only toward the engine air intake 152.

When the solenoid valve is in the first state the tunable air conduit isopen to the atmosphere, and when the solenoid valve is in the secondstate the idle air chamber is connected to engine intake vacuum. Withthe solenoid valve in the first state, vibrations of a higher frequency(i.e., engine idle frequencies) are isolated at two idle rate dipfrequencies of dynamic stiffness: a first idle rate dip by inducedhydraulic fluid oscillations in the upper fluid chamber 112 passingthrough the idle inertia track 118, in responsive combination withresilient deformation of the main resilient element and the idlediaphragm, and a second idle rate dip further by air oscillations inresponse to vibration of the idle diaphragm 126 in the tunable airconduit 140, which is open to the atmosphere. For lower frequencyvibrations (i.e., engine shake and bounce frequencies), the solenoidvalve is in the second state such that the idle air chamber 124 isevacuated by the tunable air conduit 140 being connected to a source ofvacuum via the engine air intake 152, wherein now the hydraulic fluidoscillations of the upper fluid chamber 112 travel through the bounceinertia track 120 and are damped thereby in responsive combination withthe resilient deformation of the main resilient element 110 and thebellows 136.

Advantageously, the vacuum tank 148 provides a reliable and smoothed-outsource of vacuum to the tunable air conduit when connected thereto bythe solenoid valve 142 being at the second state, such that the air inthe idle air chamber 124 is evacuated and the idle diaphragm 126 isimmobilized, as shown at FIG. 2. Advantageously further, the one-waycheck valve 150 only allows air flow (see arrow A) from the vacuumchamber 148 toward the air intake 152 of the engine. This featurepresents fuel vapors from entering into the tunable air conduit 140 andthe idle air chamber 124, whereby the idle diaphragm 120 is kept free ofcontact with fuel vapor.

The algorithm 200 of FIG. 3 exemplifies how the idle inertia track 118is resonance tuned to provide a first idle rate dip frequency of dynamicstiffness, and, in that the air in the idle air chamber 124 pulses intothe tunable air conduit 140 in response to vibration of the idlediaphragm 120, how the tunable air conduit 140 is additionally resonancetuned to provide a second idle rate dip frequency of dynamic stiffness.

First, at Block 202, determined is the double idle rate dip frequenciesof dynamic stiffness that are associated with the particular engine withrespect to objectionable vibration orders of the one time and two timeengine firing frequencies.

For example, as shown at FIG. 4A which is a graph 300 of dynamicstiffness of an hydraulic mount according to the present inventionversus engine vibration frequency, wherein the plot 302 has a first idlerate dip 304 at a frequency of 50 Hz, which is associated with a secondorder vibration of a one time engine firing frequency, and the secondidle rate dip 306 at a frequency of 100 Hz, which is associated with afourth order vibration of a two time engine firing frequency. It isinteresting to compare this graph with that of FIG. 4B of the prior artreferenced above, wherein Table II, below, provides the test parametersfor the respective plots.

TABLE II Idle Idle vacuum Vacuum Idle track Idle track diaphragm TubeTube chamber connector area, length, hardness length, diameter, volume,FIG. size, mm mm² mm shore A mm mm mm³ 4A 3.5 472 35 65 800 5.7 5121 4B3.5 472 28 41 600 4.3 2222

In carrying out Block 202, it is preferred for the determination of theidle rate dip frequencies of dynamic stiffness to be matched to the idlefiring frequencies of the engine, thereby minimizing vibration to themotor vehicle frame which can be felt by the passengers.

Next, at Block 204, the idle inertia track 118 is resonance tuned toprovide the first idle rate dip frequency of dynamic stiffness. In thisregard, the idle inertia track is dimensionally adjusted (i.e.,cross-sectional area and length) with respect to the structure of thehydraulic mount vibrationally interacting therewith, including the mainresilient element 110, the hydraulic fluid 114, the idle fluid chamber122, the idle diaphragm 126, the idle air chamber 124 and the tunableair conduit 140. This portion of the resonance tuning may be performedfor example more or less generally according to methods well known inthe prior art, as for example exemplified in U.S. Pat. No. 5,215,293.

Then, at Block 206 the tunable air conduit 140 is resonance tuned,taking account of the resonance tuning of the idle inertia trackincluding the aforementioned vibrational interactions, by dimensionaladjustment (i.e., the internal volume dimensions) to provide the secondidle rate dip frequency of dynamic stiffness, which may include anyresonance technique known in the art of resonance.

By way of example, the frequency of the second idle rate dip frequencyof dynamic stiffness is resonance tuned by empirical testing or bymathematical modeling, based for example upon the well known physics ofacoustic resonance of the tunable air conduit 140 when open to theatmosphere by the solenoid valve being at the first state.

In that the resonance tuning of the tunable air conduit 140 may nowchange the resonance tuning of the idle inertia track 118 as provided atBlock 204, inquiry is made at Decision Block 208 whether the resonancetuned first and second idle rate dip frequencies are the same as thosethat were determined at Block 202. If the answer to the inquiry is yes,then the algorithm completes. However, if the answer to the inquiry isno, then the algorithm returns to Block 204 until the resonance tunedidle rate dip frequencies are the same as the determined idle rate dipfrequencies determined at Block 202.

FIGS. 5A through 5C contrast idle rate dip hydraulic mount resonancetuning methodologies of the prior art with that of the present inventionfor minimizing the undesirable one and two times engine firingfrequencies associated, respectively, with second and fourth vibrationorders 402, 404 of an I4 IC engine, where dashed lines represent theidle air chamber vented to atmosphere, and solid lines represent theidle air chamber evacuated.

In the prior art, the idle dip compromise proceeds as follows. At FIG.5A, an attempt to provide minimized amplitude at idle rate dip A2 forthe second vibration order 402 results in an undesirable increase inamplitude at the idle rate dip A4 for the fourth vibration order 404. Anattempt to compromise, as shown at FIG. 5B, reduces amplitude at idlerate dip A4′ for the fourth vibration order 404, but this then reducesthe amplitude improvement of idle rate dip A2′ for the second vibrationorder 402. The compromise is to achieve amplitude reduction of the idlevibration of both vibration orders, but the vibration amplitude is stillhigh at the second vibration order. In the end, the prior artmethodology provides only a single idle rate dip frequency of dynamicstiffness, as shown representatively at 304′ of FIG. 4B.

The present invention, as shown at FIG. 5C, however, advantageouslyprovides simultaneous minimization of amplitude at the idle rate dip A2″for the second vibration order 402 and at the idle rate dip A4″ for thefourth vibration order 404. In the end, the methodology of the presentinvention provides a double idle rate dip frequencies of dynamicstiffness, as representatively shown at 304, 306 of FIG. 4A.

In operation as shown at FIG. 1, the solenoid valve 142 is switched tothe first state so that the tunable air conduit 140 is open toatmosphere as in FIG. 1, whereupon engine vibrations of undesirable oneand two times engine firing frequencies (and their associated vibrationorders) are isolated by operation of the induced fluid oscillations inthe upper fluid chamber 112 passing through the idle inertia track 118(the first idle rate dip), in responsive combination with the resilientdeformation of the main resilient element and the idle diaphragm, andfurther thereto by induced air oscillations passing through the tunableconduit 140 (the second idle rate dip).

In operation as shown at FIG. 2, the idle air chamber 124 is evacuatedby the solenoid valve 142 being switched to the second state, whereatthe tunable air conduit 140 is connected to engine vacuum via the vacuumtank 148, one-way valve 150 and engine air intake 152. Now the fluidoscillations of the upper fluid chamber 112 travel through the bounceinertia track 120 and are damped thereby in responsive combination withthe resilient deformation of the main resilient element and the bellows136.

To those skilled in the art to which this invention appertains, theabove described preferred embodiment may be subject to change ormodification. Such change or modification can be carried out withoutdeparting from the scope of the invention, which is intended to belimited only by the scope of the appended claims.

1. An hydraulic mount, comprising: a main resilient element; an upperfluid chamber; a top plate adjoining said upper fluid chamber; an idleinertia track formed in said top plate, said idle inertia trackcommunicating with said upper fluid chamber; an idle fluid chambercommunicating with said idle inertia track opposite with respect to saidupper fluid chamber; an idle diaphragm adjoining said idle fluidchamber; an hydraulic fluid filling said upper fluid chamber, said idleinertia track and said idle fluid chamber; an idle air chamber adjoiningsaid idle diaphragm oppositely disposed with respect to said idle fluidchamber; and an air conduit assembly comprising a tunable air conduitcommunicating with said idle air chamber, wherein said air conduit isconnectable to the atmosphere via said air conduit assembly; whereinwith the tunable air conduit open to the atmosphere, the idle inertiatrack is resonance tuned, in responsive combination with resilientdeformation of the main resilient element and the idle diaphragm, toprovide a first idle rate dip of dynamic stiffness; and wherein with thetunable air conduit open to the atmosphere the tunable air conduit isresonance tuned to provide a second idle rate dip of dynamic stiffness.2. The hydraulic mount of claim 1, wherein said hydraulic mount supportsan internal combustion engine having predetermined firing frequencies;said hydraulic mount further comprising: said first idle rate dip is ata one times firing frequency of the engine; and said second idle ratedip is at a two times firing frequency of the engine.
 3. The hydraulicmount of claim 2, further comprising: a bounce inertia track formed insaid top plate, said bounce inertia track communicating with said upperfluid chamber; a lower fluid chamber communicating with said bounceinertia track oppositely disposed with respect to said upper fluidchamber, said hydraulic fluid filling further said lower fluid chamberand said bounce inertia track; a bellows adjoining said lower fluidchamber; and a lower air chamber adjoining said bellows oppositelydisposed with respect to said lower fluid chamber, wherein said lowerair chamber is open to the atmosphere.
 4. The hydraulic mount of claim3, wherein said air conduit assembly further comprises: a solenoid valveconnected to said tunable air conduit; a vacuum tank connected to saidsolenoid valve; and a connection between said vacuum tank and an engineair intake; wherein said solenoid is switchable between a first state inwhich said tunable air conduit is open to the atmosphere, and a secondstate in which said tunable air conduit is connected to the vacuum tank.5. The hydraulic mount of claim 4, further comprising a one-way checkvalve interposed between said air intake and said vacuum tank, whereinsaid one-way check valve allows air to flow unidirectionally from saidvacuum tank to said air intake.
 6. A method of providing a double idlerate dip of dynamic stiffness for an hydraulic mount, wherein thehydraulic mount comprises: a main resilient element; an upper hydraulicchamber; a top plate adjoining the upper fluid chamber; an idle inertiatrack formed in the top plate communicating with said upper fluidchamber; an idle fluid chamber communicating with the idle inertia trackoppositely disposed with respect to the upper fluid chamber; an idlediaphragm adjoining the idle fluid chamber, an hydraulic fluid fillingthe upper fluid chamber, the idle inertia track and the idle fluidchamber; an idle air chamber adjoining the idle diaphragm oppositelydisposed with respect to the idle fluid chamber; and an air conduitassembly comprising a tunable air conduit communicating with the idleair chamber, wherein the air conduit is connectable to the atmospherevia the air conduit assembly; said method comprising the steps of:determining a first idle rate dip frequency of dynamic stiffness;determining a second idle rate dip frequency of dynamic stiffness;resonance tuning the idle inertia track to provide the determined firstidle rate dip of dynamic stiffness of the hydraulic mount when the airconduit is open to the atmosphere; and resonance tuning the tunable airconduit to provide the determined second idle rate dip of dynamicstiffness of the hydraulic mount when the air conduit is open to theatmosphere.
 7. The method of claim 6, wherein said steps of resonancetuning further comprise repeating said steps of resonance tuning untilsimultaneously the idle air track provides the determined first idlerate dip of dynamic stiffness of the hydraulic mount and the tunable airconduit provides the determined second idle rate dip of dynamicstiffness of the hydraulic mount.
 8. The method of claim 7, wherein saidsteps of resonance tuning are performed responsively in combination withresilient deformation of the main resilient element and the idlediaphragm, with the tunable air conduit open to the atmosphere.
 9. Themethod of claim 8, wherein in said steps of determining, the first idlerate dip frequency of dynamic stiffness and the second idle rate dipfrequency of dynamic stiffness are, respectively, a vibration order ofthe one and two times firing frequencies of an internal combustionengine.